A fuel cell is a device that generates electricity by a chemical reaction. Among various fuel cells, solid oxide fuel cells use a hard, ceramic compound metal (e.g., calcium or zirconium) oxide as an electrolyte. Typically, in solid oxide fuel cells, an oxygen gas, such as O2, is reduced to oxygen ions (O2−) at the cathode, and a fuel gas, such as H2 gas, is oxidized with the oxygen ions to form water at the anode. Fuel cells are generally designed as stacks, whereby subassemblies, each including a cathode, an anode and a solid electrolyte between the cathode and the anode, are assembled in series by locating an electrical interconnect between the cathode of one subassembly and the anode of another.
Anode compositions for solid oxide fuel cells (SOFCs) are typically composed of mixtures of nickel oxide (NiO) and yttria-stabilized zirconia (YSZ). During operation in a reducing (hydrogen) atmosphere, the NiO reduces to nickel (Ni) metal, which then acts as the electrically-conducting phase. For the typical mixtures composed of approximately equal-sized, spherical powders in a uniform distribution, a minimum fraction of approximately 30 vol. % Ni is required to percolate sufficient Ni metal throughout the anode microstructure to conduct electricity without excessive resistance. See N. Q. Minh, Ceramic Fuel Cells, J. Am. Ceram. Soc. Vol. 76 (3), pp. 563-588 (1993). Considering the volume loss upon reduction of NiO to Ni, 30 vol. % Ni requires approximately 42 vol. % NiO, which corresponds to a minimum fraction of approximately 45 wt. % NiO in a mixture with YSZ.
SOFC anode compositions are typically composed of as much as 70-80 wt. % NiO for several reasons. The high fraction of NiO ensures good electrical conductivity and creates microstructures with increased mechanical strength. In addition, since the decrease in volume from NiO to Ni is manifested in a porosity gain within the microstructure, increasing NiO fractions also increase the volume reduction, providing an in-situ method for creating higher porosity in the anode. However, increasing fractions of NiO create difficulties during reduction-oxidation (redox) cycles. Repeated cycling from operating conditions at elevated temperatures in reducing atmospheres to shut-down conditions at low temperatures in oxidizing atmospheres creates cyclic stress conditions in the anode microstructure due to volume changes and differences in coefficient of thermal expansion. For example, a NiO/YSZ composition containing 80 vol. % NiO will exhibit an approximately 33% redox volume change. The commonly-held lower limit of 45 wt. % NiO (30 vol. % Ni) corresponds to an approximate 18% redox volume change. Thermal stress induced by cycling between reducing and oxidizing atmospheres is a known failure mode over the lifetime of solid oxide fuel cells, and is generally referred to as redox tolerance.
Therefore, there is a need to reduce or eliminate volume changes during operation of a solid oxide fuel cell.
A constant concern in the manufacturing and operation of solid oxide fuel cells is the development of mismatch stresses between different component layers due to differences in thermal expansion coefficients. With manufacturing temperatures in the range of 1,100-1,400° C. and operating temperatures in the range of 600-1,000° C., even small differences in coefficients of thermal expansion (CTE) can generate significant cyclic stresses and cause failure in a solid oxide fuel cell stack. In general, a key criterion for choosing combinations of anode and cathode compositions is minimizing the difference in the coefficient of thermal expansion between room temperature and the manufacturing temperature or operating temperature. However, since many additional properties must be optimized for anode and cathode performance, larger-than-desired CTE differences must often be tolerated.
Therefore, there is a need to reduce or eliminate cyclic thermal stresses that develop in solid oxide fuel cells because of differences in coefficients of thermal expansion.